Improve the example InvitesTest

The InvitesTest example used a hardcoded aggregate id of 1 in each
tests, which also corresponds to the default id in the
Broadway\CommandHandling\Testing\Scenario, making it not quite so
obvious as to how to work with the real aggregate id when doing
scenario testing.

This commit changes the InvitesTest to generate a new UUID in each
test and passes it through to Scenario::withAggregateId() to give
a better (more real-world) example of the proper way to use the
Scenario class